WASHINGTON (CNN) -- U.S. military intelligence officials are urgently assessing how secure Pakistan's nuclear weapons would be in the event President Gen. Pervez Musharraf were replaced as the nation's leader, CNN has learned.

Key questions in the assessment include who would control Pakistan's nuclear weapons after a shift in power. The United States is pressuring Musharraf, who took control in a 1999 coup, not to declare a state of emergency as he faces growing political opposition.

Three U.S. sources have independently confirmed details of the intelligence review to CNN but would not allow their names to be used because of the sensitivity of the matter.

The sources include military officers and intelligence community analysts.

The assessment is part of a broader review of the military and security situation in Pakistan.

Officials say that Pakistan and its nuclear weapons are always a high intelligence priority for the United States.

The current review is a result of recent developments in that country, including the prospect that Musharraf could still declare a national emergency that would give him sweeping powers.

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Although the Pakistani government ruled out the declaration Thursday, the three sources told CNN that the United States thinks Musharraf may still impose those measures.

Musharraf was elected to president in a 2002 vote that was widely viewed as rigged. His five-year presidential term expires in November and he is seeking to retain his position as president and army chief. Pakistani elections are scheduled sometime around the turn of the year.

U.S. analysts are watching current Pakistani troop movements closely to see whether Musharraf is making any moves that could indicate he is about to impose emergency measures. It appears that in recent weeks a large number of troops left the Kashmir region to go to the tribal regions along the Afghan border, officials say.

Afghan officials have accused Pakistan of allowing Taliban and al Qaeda fighters to regroup and carve out a new safe haven along Pakistan's largely lawless northwestern frontier.

The United States has full knowledge about the location of Pakistan's nuclear weapons, according to the U.S. assessment.

But the key questions, officials say, are what would happen and who would control the weapons in the hours after any change in government in case Musharraf were killed or overthrown.

Musharraf controls the loyalty of the commanders and senior officials in charge of the nuclear program, but those loyalties could shift at any point, officials say.

The United States is not certain who might start controlling nuclear launch codes and weapons if that shift in power were to happen.

There is also a growing understanding according to the U.S. analysis that Musharraf's control over the military remains limited to certain top commanders and units, raising worries about whether he can maintain control over the long term.

The U.S. officials also say one of the key problems for the U.S. military is what restrictions on U.S.-Pakistani military cooperation could be imposed if Musharraf were to impose heavy security restrictions in his country.E-mail to a friend